A bag is a trick you take beyond your bid
Your team bids seven and takes nine. The first seven score seventy. The other two score one point each and put two bags on your pile.
Bags look harmless. A point is a point, and two extra tricks feel like a good hand.
Ten bags costs a hundred points
When your pile reaches ten, you lose a hundred points and ten bags come off. Games run to five hundred, so one bag penalty wipes out two made contracts.
This is the rule that decides most games. A team that bids honestly and makes eight contracts beats a team that bids low and makes twelve.
Underbidding is what fills the pile
A player who counts four tricks and bids three feels safe. They make the contract every time and never get set.
They also hand their team a bag every hand. Ten hands of that is a hundred points gone, and they never lost a single contract to see it happen.
Duck the trick once your bid is safe
The moment your team has the tricks it promised, stop winning. Play your lowest card in the suit that was led. Throw away your smallest card when you cannot follow.
This feels wrong, because winning a trick feels like progress. It is not. After the bid is made, every trick you take is a point now and ten points later.
Do not cut a trick you do not need
Cutting with a trump when you are void is how you take a trick nobody expected you to take. Once your bid is made, keep the trump and throw a low card from a side suit instead.
The one exception is a trick that would set your opponents. Taking that one is worth a bag.
Watch the pile, not just the score
Two teams at three hundred points are not in the same position if one of them has eight bags. That team is two loose tricks away from being back at two hundred.
Look at the bag count before you decide how hard to push a hand. On this site the bag count sits in the scoreboard above the table, next to the bid and the tricks taken, so it is visible on every deal.